variables do not need to be "declared" in the traditional sense.. for example if you want to set variable a to 12 and b to "abcd", you don't have to declare 'a' as and integer and 'b' as a string... just
a = 12; b = 'abcd'
will do.. also, as far as i know (and others correct me if i am wrong), all such variables have global scope within an .m file. usually, different functions go into different .m files (that is how i've always used it).
i've used the tf function before it has worked.. so i am not sure what it is that you are trying to do... if you are trying to plot the impulse response or something you might want to look up the 'impulse' function in matlab.. try 'help impulse' .. another function i've found useful is the filt function... you might want to look that one up as well..
anyway, hope you find this helpful..
k